The water gas shift process is well established as a means to increase the hydrogen content and/or reduce the carbon monoxide content of synthesis gases produced by steam reforming, partial oxidation and gasification of hydrocarbon and carbonaceous feedstocks. The reaction may be depicted as follows.H2O+COH2+CO2 
The reaction is mildly exothermic and a favourable equilibrium is obtained at low temperatures. However it is often convenient to operate a single shift process at elevated temperatures to generate shifted gas mixtures with low CO contents (typically <10 mole % on a dry gas basis). To achieve acceptable conversion, iron-containing catalysts have found widespread use as so-called high-temperature-shift (HTS) catalysts. These catalysts are typically provided as a particulate fixed bed in axial or radial-flow shift converters that are operated at inlet temperatures above 340° C. in order to achieve an acceptable activity. Alternatively cobalt-molybdenum catalysts, which can be used in so-called “sour shift” processes, may be operated with inlet temperatures above 220° C.
A problem arises with synthesis gases derived from partial oxidation and in particular gasification of carbonaceous feedstocks. Such synthesis gases contain hydrogen, carbon oxides and a small amount of steam but have a relatively high carbon monoxide contents, typically ≧45 mole % CO on a dry gas basis. Such “reactive” synthesis gases can, as a result of the increased amount of shifting required to generate acceptably low CO-containing product gases, cause overheating and deterioration of the shift catalyst if operated conventionally.
GB1302135 describes a multi-stage shift process wherein a gaseous feed stream comprising CO and H2O is passed over a first catalyst bed of water-gas shift catalyst at 350-1050° F. and 1-250 atm to produce a partially shifted gas mixture, a portion of which is recycled to the feed stream and the remainder cooled and passed over a second water gas shift catalyst bed disposed in the same vessel. The complicated use of multiple beds with inter-bed cooling was performed to maximise conversion of conventional synthesis gas mixtures at low steam:dry gas ratios and is not suited to reactive synthesis gases.
U.S. Pat. No. 4,161,393 describes a multi-stage sour shift process wherein a raw gas produced by the gasification of a solid fuel is split; one portion is fed to a saturator where it is saturated with water vapour and fed to a shift vessel containing a sour shift catalyst at an inlet temperature in the range 280-450° C., to produce a shifted gas mixture which is combined with a further portion of the raw gas and fed to further shift vessels at an inlet temperature in the range 300-500° C. There is no recycle of the shifted gas mixture to the inlet of the shift vessels.